Until recently, a three-cylinder piston pump driven by a crank link mechanism is commonly used in oil drillings. This pump may generate a high pressure of 20-25 Mpa as well as a flow rate up to 40 l/sec and is widely used in processes of high pressure jet-drillings, near-equilibrium drillings and directional group drillings, but it is complex in structure, heavy in weight, and difficult in transportation. In addition, the cylinders and pistons of the pump are easily worn and such a pump has a short service life, inefficiency, inconvenience in assembling and disassembling, and a non-uniform output flow. Moreover, the clearance between transmission members, such as a crank and a link, may be increased as the service time is extended and the wear of the members becomes intensified, which will aggravate the impact of movement and add the non-uniformity of the flow. This non-uniformity will cause transfer lines to jump in a drilling process, which gives an impact to well walls and is associated with stronger noises. The Chinese Utility Model No. 85203981 entitled "Slurry Pump for Oil Drilling" proposed a seven-cylinder rotary plunger pump with a plunger moved in a cylinder by a swashplate and a sloping cam plate. In comparison with the aforementioned three-cylinder plunger pump, the seven-cylinder rotary plunger pump has a smaller volume, a light weight, a uniform output flow, and a higher pressure; however, it also suffers from the easily worn plunger and cylinders, a short service life, inconvenience in maintenance, and a violent impact of movement, etc.